In recent years, a technique of manufacturing a thin film transistor (hereinafter, referred to as a TFT) over a substrate has been drastically progressing, and the application and development thereof to an active matrix display device, a thin film integrated circuit device, and the like has been advanced. In particular, a TFT using a polycrystalline semiconductor film has higher electron field-effect mobility (also referred to as mobility) than that of a TFT using a conventional non-single crystal semiconductor film; therefore, high speed operation can be performed. Hence, in a case where the TFT is used for a display device, the control of a pixel which is conventionally performed in a driver circuit provided outside a substrate has been tried to be performed in a driver circuit formed over the same substrate as a pixel.
Meanwhile, as for a substrate used for a semiconductor device, a glass substrate is considered to be more promising substrate than a single crystal semiconductor substrate in terms of cost. The glass substrate is inferior in heat resistance and easily deformed by heat; therefore, in the case of crystallizing a semiconductor film to form a TFT using a polycrystalline semiconductor film over a glass substrate, laser annealing is employed to avoid heat deformation of the glass substrate.
Specifically, a glass substrate over which a semiconductor film is formed is put on an XYθ stage, and laser beam irradiation is performed. Here, the laser beam with which the semiconductor film is irradiated is shaped by an optical system so that a beam spot shape formed on an irradiation surface becomes a linear shape. Such a laser beam is also referred to as a linear laser. When the substrate is relatively scanned with respect to laser light by moving the XYθ stage along with laser beam irradiation, innumerable crystal nuclei are generated in the semiconductor film which is completely melted by the laser beam, and crystal growth occurs from each crystal nucleus to a scanning direction of the laser beam as a solid-liquid interface is moved. In this manner, a crystal with a large grain size is formed.
However, “linear” of the linear laser does not strictly mean only “a line”, but also includes a quadrangular shape and an elliptical shape with a high aspect ratio (as a guide, an aspect ratio of 10 or more, preferably 100 or more). A beam spot formed on an object to be irradiated, is formed in a linear shape, or a quadrangular shape or an elliptical shape with a high aspect ratio in order to ensure energy density for performing sufficient annealing on the object to be irradiated. Therefore, there is no problem even when a beam spot has a plane having a quadrangular shape or an elliptical shape as long as sufficient annealing can be performed on the object to be irradiated.
The length of a linear beam in the minor axis direction is necessary to be approximately several μm so as not to generate turbulence in a melted semiconductor film. If turbulence is generated, crystal growth direction becomes random when the semiconductor film melted by the laser beam is crystallized, whereby a large grain size region is not formed in some cases. On the other hand, the length of a linear beam in the major axis direction is determined by types or output of a laser beam emitted from a laser oscillator, or types or a thickness of a semiconductor film. The output of the laser beam is approximately 20 W at the maximum in order to prevent a laser crystal of the laser oscillator from being damaged by heat. When the length of the laser in the minor axis direction is several μm, the length thereof in the major axis direction is approximately 500 μm.
As the substrate is enlarged, the number of scanning of the laser beam is increased. To perform this treatment efficiently, it is better to perform the treatment using a plurality of laser oscillators at one time, resulting in favorable productivity. However, as the number of laser oscillators is increased, it is necessary to have a place to locate the laser oscillator and a place to locate an optical system for condensing a laser beam. Further, it is necessary to suppress variation in intensity of a laser beam among the plurality of laser oscillators. A difference in crystal state after laser irradiation treatment using a plurality of laser oscillators has an influence on yield in one substrate. Further, it is necessary to adjust the position of each laser irradiation and to control beam profile precisely. As a means to solve these points, the following methods can be given.
A first method is a method for making a laser beam enter one end of an optical fiber, and propagating the laser beam by the optical fiber, thereby irradiating an object to be irradiated with the laser beam emitted from the other end of the optical fiber (for example, refer to Patent Document 1: Japanese Published Patent Application No. H6-79487).
A second method is a method for making a laser beam enter one end of a pipe using a propagate unit including a mirror in an articulated portion which connects pipes, thereby propagating the laser beam in the pipe while the laser beam is reflected by a mirror in the articulated portion (for example, refer to Patent Document 2: Japanese Published Patent Application No. H4-138892).
A third method is a method by which a laser head and an optical system for forming a linear beam are located on a Y-axis, a glass substrate is located on an Xθ stage, and a position of laser irradiation is adjusted to be used by adjusting a distance between the laser heads.